Arrays of high speed, high precision actuation devices are becoming required for a proliferating number of applications, in diverse fields. Deep space astronomical observatories may use multifaceted mirrors, each facet independently controlled by a precision actuator. Digital projection cameras manipulate a plurality of reflectors, in order to cast an image onto a projection screen.
But optical telecommunications in particular, have an urgent need for high bandwidth, low inertia, microactuated reflectors. This industry uses optical wavelength radiation in communication channels, spanning long distances via optical fiber. The fiber generally carries single mode light, but with a bandwidth such that multiple frequencies can be transmitted by a single fiber strand. Called dense wavelength divisional multiplexing (DWDM), each of the multiple frequencies carries a single channel on a specific frequency within the fiber bandwidth.
A major technical challenge for optical telecommunications is switching the optical beams from a set of input fibers to a set of output fibers. Presently, this switching is achieved by converting the optical signal into an electrical signal, switching the signal electronically to the output circuit, and then converting the electrical signal back into modulated light that is inserted into an output fiber. This procedure has many disadvantages, including high cost, complexity, large footprint, and is relatively difficult to upgrade as bandwidths and frequencies change with time.
It is desirable therefore, to accomplish this switching without converting the light into electrical signals. An array of optical switches would address this application, wherein each optical switch (a micromirror) would redirect light from one of N input optical fibers into one of M output fibers. N×M array of such switches would constitute an all-optical N×M fabric for a telecommunications fiber network.
For such high speed, high precision applications, stringent design criteria are set on the physical and mechanical properties of the actuator. It should have low inertia and low power requirements. For low cost applications, it should also be mechanically simple. These considerations have led to the miniaturization of familiar electromechanical devices, using photolithographic processing rather than machining bulk components. Formation of sub-millimeter scale electromechanical systems is now well known in the art, as micromachined electromechanical systems, or MEMS.
Micromachined solenoidal magnetic actuators are known in the MEMS art as micro-solenoid switches. Typically, a slug of magnetic material is affixed to a piston or plunger, and a coil is provided whose diameter is sufficient to admit the slug into its interior. The coil is then energized to repel or attract the slug, depending on the direction of current in the coil. The resulting linear mechanical motion is used to actuate various linear devices, such as opening and closing a switch or valve, or driving a piston. An embodiment of a linear, solenoidal micro-actuator is found for example, in Guckel, et al., U.S. Pat. No. 5,644,177 (1997), “Micromechanical magnetically actuated devices.”
Another design option is a rotary actuator. This device resembles a miniaturized electromagnetic motor, with a ferromagnetic core material deposited on the substrate and wound with an electrical coil. The core is patterned with some arrangement of gaps, into each of which protrudes a driven member which interacts magnetostatically with the flux across the gap. A plurality of such elements, when driven in the proper sequence and timing, can produce a positive torque on a freely rotating member. Magnetostatic micromotors can be used as rotary actuators by mounting the device of interest onto the moving member, i.e. the rotor. This concept is clearly described in Mehregany, et al. in U.S. Pat. No. 6,029,337 (2000), “Methods of fabricating micromotors with utilitarian features.”
However each of these MEMS devices suffer from a common drawback, which is that the actuation motion is constrained by design and fabrication considerations, to be in the fabrication plane of the device. For example with solenoidal electromagnetic actuator, such as described in the aforementioned prior art, motion of the magnetic slug is required to be in the fabrication plane of the device. For the rotary micromotor, the device is mounted on the rotor, and so rotates in the plane of fabrication of the motor.
Many applications require motion in the orthogonal plane, i.e. vertical to the original plane of fabrication of the MEMS device, and an array of such devices is needed. Examples of such applications include ailerons composed of a multitude of fluid flow diverters, actuated or pivoting in the vertical direction relative to the array. N×M optical switches require an array of independently addressable shutters or reflectors, in which the plane of actuation of the individual devices is orthogonal to the plane of the array, in order to intercept the beams of light passing over the horizon of the array. Projection cameras and multi-faceted reflectors require a tilting motion of the individual mirrors.
Therefore the MEMS fabrication substrate, cannot serve as the array plane for these applications, because the plane of actuation would be parallel to, not perpendicular to, the plane of the array.
Additional beams, gears and bearings can translate actuator motion out-of-plane, as in Ho et al., in U.S. Pat. No. 5,629,918 (1997), “Electromagnetically actuated micromachined flap.” In this invention a flap, which is the moving member of the actuator, is coupled by one or more beams to a substrate and thereby cantilevered out of the plan of the substrate. While conceptually this invention allows larger motions in out-of-plane directions, the need for multiple beams and pivots seriously complicates the design and fabrication of the device, and deleteriously affects tolerances and rigidity.
Therefore, an assembly of MEMS actuation devices, operating as an array, and with the plane of motion of each device being substantially perpendicular to the plane of the array, is not heretofore known in the art. Accordingly, there is a distinctly felt need for such an assembly of MEMS devices in a wide variety of applications, and in particular, within the optical telecommunications industry.